Along with increase in the amount of data communication in recent years, the need of a wireless communication system that achieves higher spectrum efficiency is increased. In such circumstances, the standardization of the LTE (Long Term Evolution) standard called the 3.9th generation radio communication standard has been completed and in an uplink (communication from a mobile station apparatus to a base station apparatus), DFT-S-OFDM (Discrete Fourier Transform Spread Orthogonal Frequency Division Multiplexing: also called SC-FDMA (Single Carrier Frequency Division Multiple Access), DFT-precoded OFDM, OFDM with DFT Precoding, etc.) system is used, which can easily maintain orthogonality between mobile station apparatuses at the time of multi-access and allocates a single carrier spectrum to contiguous frequencies in a frequency domain. As the background of this, there is a need for a mobile station apparatus to keep high power use efficiency of a power amplifier with limited transmit power in the uplink, and also the fact exists that the single carrier system is more suitable, which is excellent in PAPR (Peak to Average Power Ratio) characteristics.
As one of methods for improving the spectrum efficiency of the above-mentioned DFT-S-OFDM system, the frequency clipping technique that does not transmit part of a frequency spectrum in the DFT-S-OFDM system is discussed (hereinafter, referred to as Clipped DFT-S-OFDM) (see Non Patent Document 1). In Clipped DFT-S-OFDM, compared to a case where clipping is not performed, usage is available, in which transmission is performed more efficiently with less frequency bands, or a larger amount of information is transmitted using the same frequency bands, and therefore, Clipped DTF-S-OFDM is a useful technique in the current communication environment, in which tight situations of frequency resources are accelerated due to an increase in the number of users and in the amount of information.
FIG. 19 is a block diagram showing a configuration example of a transmission apparatus in the case where Clipped DFT-S-OFDM is used in uplink transmission. The transmission apparatus needs to be notified of various kinds of parameters (number of allocated resources, mapping information, modulation scheme, coding rate, etc.) used in transmission as control information by the reception apparatus before performing transmission of data. Because of this, first, the transmission apparatus extracts control information in a control information extraction unit 105 after down-converting a signal from the reception apparatus received at a reception antenna unit 101 in a radio reception unit 103. The transmission apparatus sets various kinds of parameters to be applied to transmission based on the extracted control information.
The transmission data is first error correction encoded in an encoding unit 107 and then modulated in a modulation unit 109. At this time, the coding rate of error correction coding applied in the encoding unit 107 and the modulation multi-value number applied in the modulation unit 109 are selected, respectively, based on coding rate information 1001 and modulation scheme information 1002 (also referred to as MCS (Modulation and Coding Scheme) etc. as information integrating the two pieces of information) included in control information notified from the control information extraction unit 105. The modulated signal is converted into a frequency domain signal by the DFT (Discrete Fourier Transform) in a DFT unit ill. Here, a size NDFT of an output of the DFT unit 111 (hereinafter, referred to as DFT size) is determined by allocation resource number information 1003 included in the control information output from the control information extraction unit 105. However, it may also be possible to calculate the allocation resource number information using mapping information 1005 indicative of the frequency position of an allocation resource.
Next, a clipping unit 113 clips part of the output of the DFT unit 111 based on clipping information 1004 output from a clipping control unit 115 and outputs signals at the remaining N points. Here, clipping means assuming that the clipped part has no signal and in the present specification, the clipping rate is defined as “1−(number (N) of output components of the clipping unit 113/DFT size (NDFT))” (NDFT≧N). The clipped signal is allocated to a subcarrier used in transmission in a subcarrier mapping unit 117. At this time, allocation is performed based on the mapping information 1005 given from the control information extraction unit 105 and zero is inserted into a subcarrier not used in transmission.
An IFFT (Inverse Fast Fourier Transform) unit 119 inverse-Fourier transforms a transmission signal output from the subcarrier mapping unit 117 to convert the transmission signal from a frequency domain signal into a time domain signal. After that, into the obtained time domain signal, a CP (Cyclic Prefix) (signal obtained by copying part of the rear of a symbol after IFFT) is inserted in a CP insertion unit 121. Next, the signal is up-converted into a radio frequency band signal in a radio transmission unit 123 and is transmitted from a transmission antenna unit 125.
On the other hand, in the reception apparatus, it is made possible to restore the transmission data without deteriorating the characteristics so much of the transmission signal whose part of a spectrum is clipped by using the nonlinear iterative equalization (for example, frequency domain SC/MMSE (Soft Canceller followed by Minimum Mean Square Error) turbo equalization) technique.
FIG. 20 is a block diagram showing a configuration example of a reception apparatus using frequency domain SC/MMSE turbo equalization. In the reception apparatus, first, after a signal received at a reception antenna unit 201 is down-converted in a radio reception unit 203, the CP is removed in a CP removal unit 205. The obtained parallel signal is converted by the FFT (Fast Fourier Transform) from the time domain signal into a frequency domain signal in an FFT unit 207 and is separated into a signal for each user in a subcarrier demapping unit 209. The number of components (N) of the separated frequency domain signal for each user is equal to or less than the number of output components (NDFT), of a DFT unit 223 used in the transmission apparatus, and therefore, zero is inserted into the same frequency component as the signal clipped on the transmission side in a first zero insertion unit 213 based on clipping information 2001 given from a clipping control unit 211. This is an operation to attach zero to both ends or one end of the output signal of the subcarrier demapping unit 209 and by this operation, the frequency signal having the same size as the number of output components (NDFT) of the DFT used on the transmission side is output from the first zero insertion unit 213. Here, the clipping information 2001 given from the clipping control unit 211 may be information determined in the transmission apparatus and notified as control information, or information determined by the reception apparatus.
A pilot signal for channel estimation is input into a channel estimation unit 215, which calculates a channel estimation value using the input pilot signal. Into the position of a clipped spectrum of the calculated channel estimation value, zero is inserted in a second zero insertion unit 217 based on the clipping information 2001 given from the clipping control unit 211. The zero-inserted channel estimation value is output to a channel multiplication unit 219 and an equalization unit 221. The channel multiplication unit 219 multiplies the frequency domain signal output from the DFT unit 223 by the zero-inserted channel estimation value input from the second zero insertion unit 217 and outputs the obtained signal to a cancel unit 225.
In the cancel unit 225, the frequency domain signal given from the channel multiplication unit 219 is subtracted from the frequency domain signal given from the first zero insertion unit 213, and thereby the replica of a desired signal is cancelled and residual signal components are calculated. However, in the first processing in the cancel unit 225, the signal replica is not generated, and therefore, cancel processing is not performed and the frequency domain signal given from the first zero insertion unit 213 is output to the equalization unit 221 as it is. The equalization unit 221 performs equalization processing using the output of the cancel unit 225 and the channel estimation value, which is the output of the second zero insertion unit 217, and then restores the desired signal using the signal replica, which is the output of a replica generation unit 227, after conversion into the time domain is performed by the IDFT (Inverse DFT).
Here, into the channel estimation value used in equalization processing, zero is inserted in the second zero insertion unit 217, and therefore, the reception apparatus performs equalization by handling the spectrum clipped in the transmission apparatus as if it is lost by the fall of the channel. Such processing makes it possible to correctly reproduce the signal before being clipped in the transmission apparatus.
After that, the signal output from the equalization unit 221 is demodulated in a demodulation unit 229 and subsequently, is error corrected in a decoding unit 231 and an LLR (Log Likelihood Ratio) of a coded bit is calculated. Iteration of equalization processing is determined based on the LLR in an iteration control unit 233 and in the case where processing is repeated, the LLR is output to the replica generation unit 227 in order to generate a soft replica of the signal and in the case where the iteration processing is exited, the LLR is output to a determination unit 235. The replica generation unit 227 generates a soft replica in accordance with reliability of the LLR of the coded bit. The generated replica is input to the DFT unit 223 and then input to the channel multiplication unit 219 described previously. Further, the replica generation unit 227 outputs the generated replica to the equalization unit 221 for reconfiguration of the desired signal at the time of equalization.
Such equalization processing is repeated a plurality of times based on the number of times of iteration determined by the iteration control unit 233, and finally the decoded bit is obtained by performing hard decision on the LLR of the information bit sequence in the determination unit 235.